Functions
6-220 7SA522 Manual
C53000-G1176-C119-2
Fuse Failure
Monitor
(Non-Symmetrical
Voltages)
In the event of measured voltage failure due to a short circuit or broken conductor in
the voltage transformer secondary circuit certain measuring loops may mistakenly see
a voltage of zero, which due to the load current may result in an unwanted pick-up or
even trip.
If a VTminiature circuit breaker (mcb) with correspondingly adjusted auxiliary contacts
is not available, but instead e.g. fuses are used, the fuse failure monitor may be acti-
vated. Naturally, it is also possible to use voltage transformer mcb and fuse failure
monitor at the same time.
The non-symmetrical measured voltage failure is characterized by its voltage unsym-
metry with simultaneous current symmetry. In Figure 6-117 the logic diagram of the
fuse failure monitor during unsymmetrical failure of the measured voltage is shown.
If there is substantial voltage unsymmetry of the measured values, without unsymme-
try of the currents being registered at the same time, this indicates the presence of a
non-symmetrical failure in the voltage transformer secondary circuit.
The unsymmetry of the voltage is detected by the fact that either the zero sequence
voltage or the negative sequence voltage exceed a settable value ))08!PLQ. The
current is assumed to be sufficiently symmetrical, if both the zero sequence as well as
the negative sequence current are below the settable threshold ))0,PD[.
As soon as this state is recognized, the distance protection and all other functions that
operate on the basis of undervoltage (e.g. also weak infeed tripping) are blocked. The
immediate blocking demands current flow in at least one of the phases. The distance
protection may be switched over to definite time overcurrent emergency operation if
the overcurrent protection was configured accordingly (refer to Section 6.9).
The fast blocking may not occur as long as one phase is without voltage due to a
single-pole dead time condition, as the non-symmetry of the measured values arising
in this state is due to the switching state of the line and not due to a failure in the sec-
ondary circuits. Accordingly, the fast blocking is disabled when the line is tripped
single-pole (internal information “SROHRSHQ” in the logic diagram).
If a zero sequence or negative sequence current is detected within approximately 10 s
after recognition of this criterion, the protection assumes a short-circuit and removes
the blocking by the fuse failure monitor for the duration of the fault. If on the other hand
the voltage failure criterion is present for longer than approx. 10 s, the blocking is per-
manently activated (latching of the voltage criterion after 10 s). Only once the voltage
criterion is removed by correction of the secondary circuit failure, will the blocking au-
tomatically reset, thereby releasing the blocked protection functions again.
To Next Page
Loading...
Need help?
General
